Patrick Smacchia has done some analysis on SP1 using the NDepend tool to compare different versions of assemblies. This is an interesting way to understand the large, standalone changes that we did, such as the new syndication features, but there's no easy way to distinguish whether individual changes in methods are due to fixing bugs, improving performance, or adding new features. For example, some of the serialization changes are easy to spot because they touch many methods with distinguishing names while other changes are more subtle. The work on performance is almost invisible unless you know where to look.

If there's interest, I may try to put together a list of notable WCF changes for SP1 in a few days that goes into more detail than the release announcement did.